1. Field of the Invention
The present invention relates to a printer control technique for high-speed printing.
A printer employed for a computer system, etc., employs coloring agents for a limited number of colors, such as CMY or CMYK, and forms or does not form small individually colored dots at locations on paper that correspond to pixels (for some printer models, the sizes of dots are further changed at a plurality of levels), thereby providing a pseudo-continuous tone image that appears, to the eyes of a viewer, to be a continuous tone image. Normally, final image data that are required by a printer are CMYK raster data that determine whether dots for CMYK coloring agents should or should not be formed at locations corresponding to individual pixel positions (for some printer models, the size of a dot is designated at one of a plurality of levels). Since the CMYK raster data have only two or slightly more levels of resolutions for the individual color components, in this specification such raster data are called “low-resolution” CMYK raster data.
Original image data, which are generated or are externally entered with an application used by a host computer that issues print commands to a printer, are normally high-resolution RGB data, which are represented by a host display color system that differs from a printer display color system, that specifically use an RGB display system, and for which high resolution levels, such as 256 levels, are provided for the individual color components. The original image data may be low-level data (raster data) that are represented as a set of pixel values, or they may be high-level data that are represented by a graphic function or character codes.
2. Description of the Related Art
In a conventional printing system, a printer driver, which is software that is provided for a host computer, or imaging software, which is provided for a printer, converts high-resolution RGB data for an original image into the final low-resolution CMYK data. This conversion process includes “rasterization,” for converting high-level original image data to raster data, “color conversion,” for employing a lookup table to convert RGB pixel values to CMY or CMYK pixel values, and “halftoning,” for employing error diffusion or dithering to convert high-resolution pixel values to low-resolution pixel values. For an ink-jet printer, in order to improve the image quality a so-called “interlaced” printing method, or an “overlapping” printing method, is employed whereby dots are formed in an order differing from the pixel arrangement order. The rearrangement of pixels is also performed during the above conversion process.
The above described conversion process constitutes a large load for the CPU of a host computer when the process is controlled using the printer driver, or for the CPU of a printer when control is provided by the printer. Therefore, an extended period of time is required for this processing; it is the major factor affecting the printing speed. In order to provide high-speed printing, a high-speed and high-performance CPU is mounted in a printer, such as a laser printer, to perform the above described conversion process at the printer. However, as a result the price of the laser printer is quite high. On the contrary, with an ink-jet printer, in order to reduce the price of the printer, the above conversion process is performed by a printer driver at a host computer that controls the entire process. But in this case, the printing speed attained by the ink-jet printer is fairly slow, and the length of time that the host computer is occupied is so long that other jobs can not be performed on the host side.